This is such an interesting dynamic because it is so common. Not so common that it's the norm, of course, but common enough that even if we haven't personally experienced it, it's very likely something we've seen/watched unfold.
The sister genuinely wants Joe to succeed while simultaneously wanting their mom to get off her ass about him. Further, neither mom nor sister actually have up to date information about Joe. They are both working on the model they made up about him in their heads. That particular facet (the not-at-all up-to-date model in their heads) is something almost everyone has experienced, even if it is only by a distant cousin or aunt or something like that.
Those genuine relationships that can so easily turn toxic and/or are so close to an existing toxic relationship that it can be difficult to see, let alone enumerate, the differences are what anchors this story about fiction invading fiction.